Assessing variances in the impact of intrinsic minerals on NO and N2O precursors during sludge pyrolysis

Abstract

This study proposed a novel strategy to control nitrogen oxides (NOx) precursors by directing nitrogen to adhere to the fuel-N→NH3→N2 pathway. The research aimed to illuminate the regulatory effects of CaO, Fe2O3, and their mixtures on NO and N2O precursors (NH3, HCN, and HNCO) during sludge pyrolysis, further sought to establish scientific correlations with varying temperatures and differentiate their impact. The findings indicated that unlike NH3, HNCO is generated from the cleavage of amides, sharing its origin with HCN. Notably, the sensitivities of three conditioners to different functional groups result in distinct impacts on precursor emissions. CaO interacted with –OH and N-H functional groups, releasing NH3. Fe2O3, being more responsive to C-H and C-C groups, contributed to HNCO emission. At 750 °C, a CaO-Fe2O3 mixture created CaxFeyOz, which selectively reacted with N-H and C≡N, facilitating the conversion of HNCO and HCN to NH3. While temperature is a crucial factor in the performance of CaO, its impact on Fe2O3 is primarily confined to transformation of heterocyclic and nitrile. Below 350 °C, Fe2O3 favored the evolution of nitrogen into heterocyclic compounds within tar. Conversely, above 490 °C, it inhibited the formation of nitrile in char, promoting the dehydrogenation of tar into unsaturated aromatic structures. The CaO-Fe2O3 mixtures exhibited a superposition of individual effects from CaO and Fe2O3, with CaxFeyOz stimulating N2 emission. Importantly, among three conditioners, Fe2O3 showed the most significant abatement effect on NOx precursors, primarily through nitrogen fixation in char, nitrogen enrichment in tar, and precursor oxidation. This research contributes innovative perspectives on nitrogen regulation in nitrogen-rich fuels, the utilization of NH3, and effective NO and N2O control.